A novel perforated electrode flow through cell design for chlorine generation

Chlorination remains a predominant method for disinfecting drinking water. Electrogeneration of chlorine has the potential to become the favoured method of chlorine production if costs can be lowered and chlorine generation efficiencies can be improved. A novel perforated electrode flow through (PEFT) cell design has been developed to address these problems. The electrodes were made from low-cost graphite sheets and stainless steel mesh and separated by a non-conducting fabric membrane. This electrode configuration allows reduction of electrode separation to 0.1 mm or less, minimizing cell resistance and increasing electrical efficiency. The new PEFT configuration generates hypochlorite from a 0.5 mol L⁻¹ brine at a current efficiency of better than 60%. As an inline in situ device, it produces chlorine concentrations known to be sufficient to disinfect water, from chloride concentrations as low as 0.004 mol L⁻¹ (available in most natural waters) by a single pass of the water through the cell operating at 11 V. The possibility of a portable device operated by a 12-V battery is indicated.     

Mechanism of the chlorine evolution on a ruthenium oxide/titanium oxide electrode and on a ruthenium electrode

The mechanism of the chlorine evolution on titanium electrodes coated with a layer of ruthenium oxide and titanium oxide under different experimental conditions, and on a ruthenium electrode, both in acidic chloride solution, has been investigated. Potentiodynamic current density—potential curves were recorded as a function of the time anodic pre-polarisation, the composition of the solution and the temperature. Moreover, potential decay curves were determined. Theoretical potential decay curves were deduced for both the Tafel reaction (2 Cl_ad→Cl₂) and the Heyrovsky reaction (Cl^? + Cl_ad → Cl₂ + e^?) as a rate determining step in the formation of molecular chlorine. They were compared with those found experimentally. The influence of possible diffusion of atomic chlorine out of the electrode was also taken into consideration. It was found that for all the electrodes investigated, molecular chlorine is formed both at anodic polarisation and on open circuit according to the Volmer—Heyrovsky mechanism, where the Heyrovsky reaction is the rate-determining step. The transfer coefficient is 0.5 for the chlorine evolution at an “ideal” ruthenium oxide titanium oxide electrode and at a ruthenium electrode.     

Aqueous polysulphide flow-through electrodes: Effects of electrocatalyst and electrolyte composition on performance

Porous electrodes are required to achieve satisfactory performance of the aqueous sulphide/polysulphide redox couple in energy conversion and storage applications. A flow cell for testing flow-through porous electrodes was constructed and operated. The effects of electrode material, temperature, flow rate, and electrolyte composition were studied. Catalytic electrode surface layers of Co and MoS₂ demonstrated performances which were more than adequate to meet a design goal of 10–20 mA cm^–2 at less than 50mV overpotential. Flow rate variation had only a small effect on the current density-overpotential behaviour, whereas raising the temperature and/or adding dimethylformamide to the electrolyte had much larger effects. These observations are consistent with steady-state results obtained on rotating disc electrodes.     

A membrane free electrochemical cell using porous flow-through graphite felt electrodes

This work describes the development and characterisation of an electrochemical cell which can be used to give high reactant conversion without the need for a membrane. The undivided cell uses two high porosity flow-through graphite felt electrodes, with the products flowing through the back of each electrode. A series of tests have been conducted using an equimolar mixture of potassium hexacyanoferrate (II) and potassium hexacyanoferrate (III) to characterise the cell design and identify suitable operating conditions. It has been shown that high reactant conversion (in excess of 90%) can be achieved for high concentrations of redox species at low flow rates (superficial velocities of around 0.1 mm s⁻¹). However, the cell voltage required to achieve high conversions increased with increasing concentration.     

The influence of electrode porosity and temperature on electrochemical gas evolution at platinum and rhodium

The influence of electrode porosity and temperature on the rate of electrochemical gas evolving processes (H₂, Cl₂, O₂) was investigated. The experiments were carried out at electrodes with small pores (3 nm) and at smooth electrodes. To understand the results the hydrogen evolution process was used for detailed investigations. It was shown that the pores are only effective if the gas evolving process is an irreversible one (as with oxygen). The pores do not operate in the case of hydrogen and chlorine evolution. An explanation of this different behaviour is given. The temperature dependencies of the overvoltages of the chlorine and hydrogen processes are in contrast. An increase of hydrogen overvoltage with rising temperature is not yet fully understood. It can be stated, however, that the effectiveness of the hydrogen transport from the electrode surface into the bulk of solution decreases with increasing temperature.     

Electrochemical intercalation of aluminium chloride in graphite in the molten sodium chloroaluminate medium

Aluminium chloride intercalation in graphite was studied by anodic oxidation of compacted graphite (rod) and graphite powder electrodes in sodium chloroaluminate melt saturated with sodium chloride at 175 °C. The studies carried out by employing both galvanostatic and cyclic voltammetric techniques had shown that the intercalation reactions take place only beyond the chlorine evolution potential of +2.2 V vs. Al on both the electrodes. The extent of intercalation reaction was directly related to the anodic potential and probably to the amount of chlorine available on the graphite anodes. In the case of graphite powder electrode, a distinctly different redox process was observed at sub-chlorine evolution potentials and this was attributed to the adsorption of chlorine on its high surface area. This finding contradicts a report in the literature that the intercalation reactions occur at potentials below chlorine evolution in the chloroaluminate melt.     

Electrochemical processes in the photoelectrolysis of nicotinamide adenine dinucleotide on the chlorophyll electrodes

The photoelectrolysis, in which redox compounds are electrolysed on a pair of photo-excitable electrodes by supplying photo—energy in place of electric energy, has been performed. The photo—excitable electrodes were prepared by coating a platinum plate with a thin layer of a chlorophyll—quinone composite. These electrodes were called chlorophyll electrodes. The chlorophyll electrode of chlorophyll—naphthoquinone composite worked as a cathode and that of chlorophyll—anthrahydroquinone composite as an anode when they were illuminated. The chlorophyll electrode of chlorophyll—naphthoquinone composite was characterized by an electrochemical behavior of p-type semiconductor electrode. Reduction of nicotinamide adenine dinucleotide (NAD⁺) was carried out on the chlorophyll electrode under illumination at various controlled electrode potentials. NAD was reduced at extremely noble electrode potentials are compared with the reductive potential of NAD⁺ to NADH. Electron transfer accompanied with the photoelectrochemical reactions is discussed.     

Novel graphite/TiO2 electrochemical cells as a safe electric energy storage system

A graphite/TiO₂ full cell has been developed as a new safety energy storage system using a highly safety process. The crystal structures of the anatase TiO₂ electrode have been investigated with respect to the performance of the electrodes. Due to the large anion intercalation into the graphite positive electrode, the possible charging potential can be raised to around 5.3 V against the Li/Li⁺ electrode, which is a higher charging voltage than lithium-ion batteries (maximum voltage is around 4.3 V vs. Li/Li⁺). In situ XRD measurements have been carried out on both the cathode and anode electrodes of the graphite/TiO₂ cell during the charge process to elucidate the intercalation mechanism.     

Oxygen and chlorine electrodes on semiconductive SnO2 in molten LiCl-KCl eutectic

The oxidation of oxide (O^2–) and chloride ions were studied at tin oxide electrodes in molten LiCl-KCl eutectic at 450° C using voltammetric techniques. The polarization characteristics of the oxide semiconductor electrode were compared with that of Pt in the case of the oxygen electrode and with that of glassy carbon in the case of chlorine evolution. Both electrode reactions on tin oxide were found to be somewhat less reversible compared with the results of Pt and GC. It is suggested that the observed irreversibility is related to the electron tunnelling process through the space charge barrier formed at the semiconductor surface.     

The feasibility of boron removal from water by capacitive deionization

We report on the possibility of removing boron (in the form of boric acid) from water by electrochemical means. We explore capacitive de-ionization (CDI) processes in which local changes in pH near the surface of high-surface-area activated carbon fiber (ACF) electrodes during charging are utilized, in order to dissociate boric acid into borate ions which can be electro-adsorbed onto the positive electrode in the CDI cells. For this purpose, a special flow-through CDI cell was constructed in which the feed solution flows through the electrodes. Local pH changes near the carbon electrode surface were investigated using a MgCl₂ solution probe in three- (with reference) and two-electrode cells, and described qualitatively. We show that, to a certain extent, boron can indeed be removed from water by CDI.     

Bifunctional electrodes for an integrated water-electrolysis and hydrogen-oxygen fuel cell with a solid polymer electrolyte

An alternative concept of an integrated water electrolysis/hydrogen-hydrogen fuel cell using metal electrocatalysts and a solid polymer electrolyte is described. Instead of operating both electrodes as hydrogen and oxygen electrodes respectively the electrodes are used as oxidation and reduction electrodes in both modes of operation. A more suitable selection of electrocatalysts and an improved cell design are possible; both can increase the efficiency of the cell considerably. New results on the electrocatalytic activity of various noble-metal containing catalysts with respect to both oxygen evolution and hydrogen oxidation in a proton exchange membrane-cell at 80°C are reported. Kinetic data derived from Tafel plots of the oxygen evolution polarization curves agree closely with those of experiments with aqueous sulphuric acid electrodes. This agreement allows the determination of kinetic parameters for electrocatalysts difficult to prepare in solid smooth electrodes but easy to be made into porous deposits. Polarization curves of the hydrogen oxidation reaction clearly indicate a relative activity rating of the studied catalysts. In cycling tests the lifetime stability of the new bifunctional oxidation electrode was determined. Polarization data obtained under these conditions agree with those obtained in earlier experiments where electrodes were exposed to only one type of oxidation reaction. During a test of 10 cycles (30 min of electrolyser and 30 min of fuel cell mode each) no changes in the electrode potential were observed. With the conventional cell design employing a hydrogen and an oxygen electrode both catalyzed with platinum and a current density of 100 mA cm^–2 a storage efficiency of 50% was calculated; with the alternative concept of oxidation and reduction electrodes and selected oxidation catalysts this was improved to 57%. With further improvements these efficiencies seem possible even at current densities of 500 mA cm^–2.     

Electrochemical behaviour of the chloride/chlorine system at platinum electrodes in acetonitrile solutions

The electrochemical behaviour of chlorine and chloride solutions in acetonitrile at platinum electrodes has been studied. The rotating disk electrode technique was applied in a temperature range of −6 to 30 ± 0·1°C. The over-all reaction Cl₂ + 2 e⁻ ? 2 Cl⁻ undergoes a two-electron exchange per mol of chlorine and only one anodic and one cathodic wave are found.     

Electrochemical detoxification of waste water without additives using solid polymer electrolyte (SPE) technology

Ion exchange membranes as solid polymer electrolytes (SPE) facilitate the electrochemical detoxification of waste water without addition of supporting electrolyte. Cation exchange membranes as H⁺ ion conductors or anion exchange membranes as OH^? ion conductors were used in combination with different electrode materials. A variety of cell configurations were investigated which differ in the direction of the electro-osmotic stream (EOS). This is a characteristical property of SPE technology, caused by the solvation shells of the ions during their migration through the membrane. Dependent on cell configuration mass transfer at the electrodes can be hindered or enhanced by EOS. In the latter case it is appropriate to increase EOS by preparation of Nafion^® membranes in order to decrease energy consumption per m³ waste water. Using a perforated membrane, which operates in this case only as ion conducting solid polymer electrolyte but not as cell separator, flow rates through the cell can be adjusted independent of the EOS and a further decrease of energy consumption is possible. The best results were obtained using anodic oxidation followed by cathodic reduction: 2-chlorophenol as example compound was destroyed almost completely and more than 80% of the chlorine was mineralized to chloride ions. By-products were detected in very low amounts, less than the remaining traces of 2-chlorophenol.     

Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochemical reactivity of boron doped diamond (BDD) electrodes. A comparative study has been carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown boron doped diamond electrodes. The effect of doping a pure Csp³ diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. In order to establish the effect of doping on a pure Csp³ diamond electrode, the amount of graphitic domains was increased by means of HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp² contribution of the electrode.The presence of graphitic domains can be correlatively associated with the presence of kinetically active redox sites. The electrochemical reactivity of boron doped diamond electrodes shows a distribution of kinetic constants on the whole surface of the electrode corresponding to different active sites. In this paper, we have studied by cyclic voltammetry and electrochemical impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochemical “pulse cleaning” procedure is proposed to remove them.     

Distribution of mass transfer over a 0.5-m-tall hydrogen-evolving electrode

In general, technical vertical electrolysers for the production of chlorine, hydrogen and oxygen are high and have a short cathode-anode distance. Up to the present, few results on mass transfer to gas-evolving electrodes in these industrial cells have been published. The mass transfer experiments were carried out in a divided cell for a vertical hydrogen-evolving platinum electrode in a solution containing 1 M KOH, 0.1 M KCN and 0.008 M AgCN, where the Ag(CN) ₂ ^– complex ion was used as the indicator ion. The platinum electrode was divided into 20 segments, each with a height of 24 mm and a width of 20 mm. Subsequent segments were separated by a 1-mm-thick Perspex layer. The height of the platinum electrode was 0.50 m. It has been found that the Ag/Ag(CN) ₂ ^– redox couple in an alkaline cyanide solution is very useful in determining mass transfer to a hydrogen-evolving electrode. When no hydrogen bubbles are formed at the working electrode, the mass transfer coefficient decreases at a decreasing rate as the distance from the leading edge of the working electrode increases. For a hydrogen-evolving electrode, however, it has been found that the mass transfer coefficient to the topmost 0.40 m of the working electrode is practically constant. For the entrance part of the working electrode, about 0.10 m in length, the dependence of the mass transfer coefficient on the distance to the leading edge of the electrode is complicated. The mass transfer coefficient and the mass transfer enhancement factor for the topmost 0.40 m of the hydrogen-evolving working electrode are given by complex correlations as a function of the current density required for hydrogen evolution and of the flow rate of solution. The effect of viscosity increase of the solution, caused by addition of a polymer, has been investigated. It has been found that the mass transfer coefficient decreases with increasing viscosity of solution in both cases, namely with and without gas bubble evolution, and that the mass transfer enhancement factor is independent of the viscosity of solution.Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

Removal of arsenic from drinking water by the electrocoagulation using Fe and Al electrodes

A novel technique of electrocoagulation (EC) was attempted in the present investigation to remove arsenic from drinking waters. Experiments were carried out in a batch electrochemical reactor using Al and Fe electrodes with monopolar parallel electrode connection mode to assess their efficiency. The effects of several operating parameters on arsenic removal such as pH (4–9), current density (2.5–7.5 A m⁻²), initial concentration (75–500 μg L⁻¹) and operating time (0–15 min) were examined. Optimum operating conditions were determined as an operating time of 12.5 min and pH 6.5 for Fe electrode (93.5%) and 15 min and pH 7 for Al electrode (95.7%) at 2.5 A m⁻², respectively. Arsenic removal obtained was highest with Al electrodes. Operating costs at the optimum conditions were calculated as 0.020 € m⁻³ for Fe and 0.017 € m⁻³ for Al electrodes. EC was able to bring down aqueous phase arsenic concentration to less than 10 μg L⁻¹ with Fe and Al electrodes. The adsorption of arsenic over electrochemically produced hydroxides and metal oxide complexes was found to follow pseudo second-order adsorption model. Scanning electron microscopy was also used to analyze surface topography of the solid particles at Fe/Al electrodes during the EC process.     

High efficiency hypochlorite generation

The oxidation of chloride ion at DSA anodes gives higher yields of hypochlorite than were obtained at Pt or graphite electrodes.It is also shown that the problem of hypochlorite reduction at the cathode in undivided cells can largely be overcome by use of a ‘reduced area’ cathode. These two improvements have both been included in a new cylindrical hypochlorite cell capable of a 70% conversion of a 0.048 mol. dm^?3 NaCl solution to NaOCl with a production rate of 50 gh^?1 ‘active chlorine’.     

Effect of current density and sulfuric acid concentration on persulfuric acid generation by boron-doped diamond film anodes

This research investigated the effects of current density and sulfuric acid concentration on the rates of persulfate generation by boron-doped diamond film anodes. Also investigated was the maximum conversion of sulfate to persulfate that could be achieved from electrolysis of sulfuric acid. Experiments were performed in batch systems using a rotating disk electrode (RDE) and a flow-through reactor with parallel plate electrodes. Both the RDE and flow-through experiments showed that there was a linear relationship between persulfate generation rates and current density. Persulfate generation rates became current limited at sulfuric acid concentrations of 2.25 M and above; however, Faradaic efficiencies under current-limited conditions were only ~60 %, and were only weakly dependent on the current density. Persulfate generation rates in the flow-through reactor showed similar dependencies on current density and sulfuric acid concentration as those in the RDE reactor, but were 20–50 % lower. Acid catalyzed and thermal decomposition of persulfate limited the maximum conversion of sulfate to persulfate. A maximum fractional conversion of 78 % was achieved using an initial sulfuric acid concentration of 0.77 M. Surprisingly, this value was independent of the current density over the range of 100–300 mA cm^?2.     

Transient analysis of a flow-through porous electrode at the limiting current

The equations governing the transients in the concentration, current and potential response of a porous flow-through electrode at the limiting current for a single reactant in a well-supported electrolyte have been solved. It was assumed that a potentiostatic step was applied to an electrode with a uniform feed concentration. The dispersive flux of reactant was assumed to be negligible but double-layer charging effects were taken into consideration. If the double-layer time constant is much less than the fluid residence time (νC/?? ? 1), it is quantitatively shown that the capacitive current may be neglected in interpreting the current-time response of the electrode when examined in the fluid residence time frame. If the entire electrode is to operate at the limiting current, it is quantitatively shown that the solution phase ohmic drop can become significant early in the transient such that secondary reactions may become important. The ability to interpretI versust data in terms of the limiting current species mass transfer coefficient is removed under these conditions. The results support the qualitative arguments made by Newman and Tiedemann in their comprehensive review article on porous flow-through electrodes. Finally, it is shown that ln(Faradaic current) versust can be approximately linear in a limited time span, although no useful information can be obtained from such a plot.     

The recovery of chlorine from hydrogen chloride Part 1: new method using a molten salt as the electrolyte

A new method of recovering chlorine from by-product hydrogen chloride is proposed and developed. Hydrogen chloride gas is led through a carbon pipe to a gas diffusion-type porous carbon cathode, which is immersed in a molten salt of lithium chloride (58 mol%)-potassium chloride (42 mol%) at 400°C. A graphite anode is immersed in the same electrolyte. By the direct electrolysis of gaseous hydrogen chloride, hydrogen is obtained from the cathode and chlorine is obtained from the anode. Bench scale tests were also carried out.The current capacity of the cell is 20 A. The cell voltage is 4.9 V at 20 A (500°C, electrode distance 3.8 cm) and in this case, more than 65% of it is the ohmic loss. Current efficiency is more than 90%. It can be concluded that this method is very promising.